In many mechanical applications, for instance in some agricultural machines, it is necessary to transmit motion between a drive shaft and a driven shaft, and in some cases the motion transmission ratio shall be variable. In so-called mixer-wagons, or silage cutters, or total mixed ration wagons, particular problems occur in the mechanical power transmission with variable number of revolutions. The mixer-wagons are used for preparing and distributing fodder for ruminant farm animals. The mixer-wagons can be self-propelled or towed. The towed mixer-wagon is coupled to a tractor, through which it is moved from the area where the fodder is loaded to the area where the fodder is distributed to the animals.
The mixer-wagon comprises, in combination: a tank, where the fodder is loaded and mixed, one or more mixing augers arranged inside the tank, a door for opening the tank and allowing the fodder unload, and a conveyor system, for instance a conveyor belt, for distributing the fodder directly in the mangers. The augers may have vertical or horizontal axis.
The operation of the mixer-wagon is subdivided into three steps:                the step of loading and mixing fodder inside the tank;        the step of moving, required to achieve the cowshed;        the step of distributing fodder to the animals.        
For every step, the auger speed shall be adapted to the type and quantity of mixed product. Moreover, the work speeds during the three steps are significantly different from one another. The speed required for the unloading step is almost twice the speed for the mixing step.
More particularly, during the loading and mixing step the mixer-wagon is loaded with different products and fodders of different sizes and consistencies, thus requiring different mixing speeds. Sometimes higher speed is required to chop the loaded material (for example when a bale of hay is put in the mixing tank), while sometimes lower mixing speed is required for having a more homogeneous product. Once the fodder has been loaded an mixed, the mixer-wagon shall be moved towards the distribution area, typically a cowshed. During the moving step, a slow mixing speed is required in order to avoid the material contained in the tank conglomerating, otherwise, the subsequent restart of the augers when the tank is full could be critical due to the high torque peaks generated in the mechanical transmission.
During the distribution step, the mixer-wagon moves along the cowshed, distributing fodder in the mangers in front of the animals. In this step, fodder shall be distributed uniformly along the manger, in order to have the same amount of fodder in front of each animal.
The tank empties through the discharge door due to centrifugal force. In order to have a uniform distribution without the need for continuously adjusting the tractor speed, it is necessary to have a constant material flow, and it is therefore necessary to increase the auger speed as the material quantity in the tank decreases.
Currently, in order to activate the augers the mixer-wagons are provided with a mechanical transmission taking power from the power take-off of the tractor and comprising the following components:                one or more cardan shafts to connect the mixer-wagon to the tractor power take-off and to transmit power to the augers;        a two- or three-speed gear box to vary the speed of the augers in the various work steps;        an actuating box for each auger.        
Currently, various solutions are marketed based on two-speed mechanical gear boxes, requiring to interrupt power transmission in order to engage a gear. These mechanical gear boxes have some disadvantages in the various steps. In particular, during chopping it is possible to adjust the work speed only by changing the rotation speed of the tractor endothermic engine, with consequent inefficiencies as regards the fuel consumption. During the moving step, the mixing augers are still as there is not a gear allowing a low speed. Anyway, independently of the moving step, before or during the distribution step the power transmission shall be interrupted, thus stopping the rotation of the members of the mixer-wagon to engage the high-speed gear. The complete shutdown of the mixer-wagon members makes the restart critical. In fact, the mixer-wagon is now full of fodder that, during the steps when the augers are still, could have formed a conglomerate mass. Under these conditions, very high torque peaks are generated during high-speed start-up, that are dangerous for all components of the mechanical transmission of the mixer-wagon.
Torque peaks often cause the activation of the torque limiters provided to protect the mechanical transmission of the mixer-wagon. The torque limiters protect against breakages, but their intervention entails time waste and inefficiencies.
Transmissions are also known, provided with power-shift or shift-tronic boxes, allowing to engage gear without interrupting power transmission. However, also these known arrangements are limited to only two or three discrete work speeds, and therefore it is necessary to vary the speed of the endothermic engine of the tractor to adequate the speed of the augers to the different conditions of the material.
There is therefore a need for developing a system comprising a mixer-wagon for preparing and distributing fodder combined with a mechanical power transmission unit that completely or partially overcomes one or more of the drawbacks of the prior art.